Preheat oven to 350 degrees F (175 degrees C).
Rinse the mushrooms. Remove and discard stems. Place mushroom caps in a large bowl. Cover with lemon juice.
In a medium saucepan, boil the water. Pour the water over the mushroom caps. Drain after 1 minute and pat dry.
Using a blender or food processor, thoroughly blend the onions, celery, sour cream, cream cheese, bread crumbs, lemon juice, garlic powder and salt. Transfer the mixture to a medium bowl and gently mix in the crabmeat by hand.
Fill the mushroom caps with the blended mixture. Top each cap with a dash of paprika.
Bake in the preheated oven 7 to 10 minutes, until lightly brown.
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